The present invention relates generally to the fabrication of semiconductor integrated circuits, and more particularly to the fabrication of very large-scale integration (VLSI) circuits.
Recent developments in MOS VLSI integrated circuits have resulted in integrated circuits capable of operating at higher speeds and characterized by reduced-size geometries and greater packing densities. This trend toward increasingly complex MOS devices has pushed the limits of existing materials and fabrication processes. As geometries have shrunk to one micron and less, circuit densities have increased, contact holes have become smaller, and line widths have become narrower. The resistance of the elements in the device structure increases along with the RC time constants, thereby limiting the overall speed of the device.
One proposed solution to this problem has been the use of refractory metals, such as tungsten and their silicides, as a high-conductive, low-resistance, and highly reliable interconnect material. The use of these materials provides an alternate to polysilicon for first-level interconnections and gate electrodes and an alternate to aluminum for multilevel interconnects, and a way of planarizing contacts and via holes without the need for troublesome sloped contact etching.
One of the most difficult areas in VLSI multilevel metallization technology is topography. Planarization of a certain amount is required to overcome the topography created by the underlying structures. The achievement of desired planarization, however, typically requires a precise process control, increased process complexity, and a reduction in product yield.
In a conventional process used to fabricate a multilevel MOS integrated circuit, a dielectric layer is deposited on a surface of a substrate, a contact hole is formed in the dielectric, and a first metal layer is deposited and then etched. A second dielectric layer (interdielectric) is then deposited over the metal and a photoresist layer is deposited over the second dielectric layer. The structure is then subjected to a blanket etch back in an attempt to achieve planarization of the interdielectric. A via hole is then formed in the interdielectric to the upper level of the first metal layer. A second metal layer is then deposited and etched and extends through the via hole to contact the first metal layer. Thereafter, a passivation layer is deposited and etched.
There are, however, numerous problems associated with this conventional multilevel process, such as severe topography resulting in metal step coverage problems and residual metal Problems, which may cause electrical opens for the interconnects and intralevel electrical shorts between adjacent interconnects. Other drawbacks associated with this conventional process include electromigration on narrow interconnects (particularly when aluminum is used), and the need for a metal overlap around the via and contact holes (dog-bone structure) which result in the reduction of device packing density and in a degraded circuit performance.